The present invention relates to semiconductor memory devices and memory systems. More particularly, the invention relates to a multi-bit data memory system and read method.
Contemporary electronic devices, particularly mobile devices such as MP3 players, PMPs, mobile phones, notebook computers, and PDAs, rely on data storage devices implemented with volatile and/or non-volatile memory devices. Mobile devices require increasingly greater data storage capacities in order to provide various functions, such as video playback, etc. In order to meet the demands for increased data storage capacity, multi-bit memory devices (i.e., memory devices capable of storing at least 2-bit data per constituent memory cell) have replaced single bit memory devices. Exemplary multi-bit memory devices storing multi-bit data are disclosed, for example, in U.S. Pat. Nos. 6,122,188; 6,075,734; and 5,923,587, the collective subject matter of which is hereby incorporated by reference.
If 1-bit data is stored in a memory cell, said memory cell will exhibit one of two threshold voltage distributions. That is, the memory cell may be placed in one of two states associated with data values of 1 and 0, respectively. On the other hand, if 2-bit data is stored in a memory cell, said memory cell will exhibit one of four threshold voltage distributions. That is, the memory cell may be placed in one of four states associated with data values 11, 10, 00, and 01, respectively. FIG. 1 is a conceptual illustration of threshold voltage distributions corresponding to four data states.
Constituent threshold voltage distributions should be closely controlled to allow each of the threshold voltage distributions to coherently exist within a defined threshold voltage window. One control method successfully employed to accomplish this goal is a programming method commonly referred to as the incremental step pulse programming (ISPP) scheme. According to the ISPP scheme, a programmed threshold voltage may be moved by defined increments with respect to a given threshold voltage distribution using sequence of program loops. Smaller ISPP programming increments generally allow more accurate definition of a threshold voltage distribution. Careful control of respective threshold voltage distributions allows better voltage margin definitions between data states. However, smaller ISPP programming increments also extend the amount of time required to program a memory cell to a desired state, and longer data programming cycles are generally undesirable. Accordingly, the size of ISPP programming increments must be weighed against programming time.
Even if the ISPP scheme is used, a threshold voltage distribution for each data state may expand from its defined threshold voltage distribution window due to a number of factors. For example, as illustrated by the dotted lines 10, 11, 12, and 13 in FIG. 1, each threshold voltage distribution may expand due to, for example, coupling between adjacent memory cells during programming. This coupling is called electric field coupling or F-poly coupling.
Referring to FIG. 2, it is assumed that memory cell MCA has already been programmed into one of four data states, and that memory cell MCB is currently being programmed. Under these assumptions, electrical charge accumulates on the floating gate FG of memory cell MCB as it is programmed. At this occurs, an electric potential between the floating gate FG of adjacent memory cell MCA and the floating gate FG of memory cell MCB correspondingly increases. The resulting increased threshold voltage remains due to coupling between the adjacent floating gates even after programming of the memory cell MCB is complete. Here, the memory cell MCB includes memory cells placed in a word line direction and/or a bit line direction with respect to the memory cell MCA. Due to this coupling, the threshold voltage of the previously programmed memory cell MCA is increased, and as a result, each threshold voltage distribution expands (or broadens) as illustrated by the dotted lines 10, 11, 12, and 13 in FIG. 1. As the threshold voltage distribution for each data state broadens, respective voltage margins between data states are reduced, and read margin is reduced.
One technique for resolving the expansion of threshold voltage distributions due to this coupling phenomenon is disclosed, for example, in U.S. Pat. No. 5,867,429, the subject matter of which is incorporated by reference. Additional background discussion regarding the coupling phenomenon may also be had by reviewing Korean Patent Publication No. 0683858 dated Feb. 9, 2007.
Considering the above description, there are difficulties in obtaining sufficient read margin between data states of multi-bit memory cells since corresponding threshold voltage distributions tend to expand due to electric field coupling/F-poly coupling. As a result, it can become difficult to accurately determine the data state of a programmed memory cell. Conventional remedies to this problem tend to increased the overall size and layout area of memory cell arrays incorporating multi-bit memory cells, and cut against ongoing efforts to further reduce the size of memory systems used in contemporary electronics.